This invention relates to processes of coagulating aqueous suspensions of coagulatable material using water-soluble polymeric coagulants. The aqueous suspensions can be true suspensions of suspended material or they can be liquors containing colloidally dispersed material. The suspended or dispersed material is usually solid.
Aqueous suspensions are usually clarified by the addition of one or more water-soluble organic polymers that are called coagulants or flocculants. These terms tend to be used rather inaccurately. In this specification, we use the term "flocculant" to denote a high molecular weight polymer that achieves its effect (i.e. flocculation) primarily by a bridging mechanism, with the result that they are sometimes referred to as bridging flocculants. We use the term "coagulant" to indicate a lower molecular weight, highly ionic, material that achieves its effect (i.e. coagulation) primarily by adsorbing onto the particles of the suspension and changing the surface charge on them, with little or no bridging effect between particles. As a result, the coagulants are sometimes referred to as charge neutralisation coagulants.
The coagulants have low molecular weight and because of this, and their high ionic content, they have low solution viscosity and dissolve easily into water. Typically, the molecular weight of coagulants is never more than around 1.5 million and it is often below 1 million, and indeed when the polymer is anionic it is usually below 0.5 million. In terms of intrinsic viscosity, IV is usually below 3 dl/g and generally below 2 dl/g.
Throughout this specification, molecular weights are the values obtained by gel permeation chromotagraphy and intrinsic viscosities are the values obtained using a suspended level viscometer at 25.degree. C. in 1 molar sodium chloride aqueous solution buffered to pH 7.0.
Because of the low solution viscosity of these polymers, it is possible to provide aqueous concentrates that have a satisfactory combination of viscosity and polymer concentration. These concentrates are very easy to use since they can be dosed directly into the suspension that is to be treated or can easily be diluted in-line with dilution water to form a dilute solution that can then be dosed into the suspension.
Many methods have been described in the literature for making solid forms of water soluble polymers of various molecular weights. Solid forms do, of course, have the advantage that they do not necessitate the transport and packaging of large amounts of water and so can be more convenient, especially to the manufacturer, than aqueous concentrates. However, they have not been adopted widely and solid coagulants only constitute a very small proportion of the total polymeric coagulant market. For instance, an important coagulant is polydiallyldimethyl ammonium chloride, and large amounts of this are supplied worldwide. However only one grade is available in solid form, namely Percol 368 and Magnafloc 368, from Allied Colloids Inc. and Allied Colloids Limited respectively and this constitutes only a small proportion of the total sales of this polymer.
A solid grade coagulant must, of course, be in solution form before it can function as a coagulant and, heretofore, this has necessitated the user providing dissolution make-up equipment. The convenience and economy to the manufacturer of supplying a solid, without the need to package and transport water, has therefore tended to be outweighed by the user preferring to obtain what was considered to be equivalent performance but without the inconvenience of having to provide make-up equipment.
An entirely different situation applies to flocculant polymers since these are always much higher molecular weight and many of them tend to dissolve only very slowly into water. For instance flocculant polymers generally have molecular weights of at least 4 million and usually at least 5 million, and frequently above 10 million. Intrinsic viscosity is generally above 5 dl/g and frequently above 10 dl/g, especially with the anionic and non-ionic polymers.
Because of the high molecular weights, flocculant polymers have very high solution viscosities and so it is not possible to supply handlable concentrates having adequate polymer concentrations. Accordingly, the flocculants generally have to be supplied as powders or as dispersions in oil. The polymer then has to be dissolved into water. The rate of dissolution depends upon ionic charge and molecular weight. Some dissolution can occur with some polymers within a few minutes, but many of the polymers take at least an hour to go to full solution. Such polymers include, for instance, many of the polyacrylamides. Since the flocculant polymers cannot exert their bridging effect until they are in proper solution, it is therefore conventional for the user to install sufficient make-up apparatus to ensure that the flocculant can be truly dissolved before it is dosed into the suspension that is to be treated. Such make-up apparatus generally has to involve a storage vessel that can hold the flocculant for at least an hour while it goes into true solution.
Although this is true of normal flocculation processes, a few exceptions to this general rule have been proposed in the literature.
For instance in JP-A-48084776 and JP-A-49049802 high molecular weight flocculant is added as powder directly into a sludge that is being transported or pumped into a pit. In each instance the flocculant is partially hydrolysed polyacrylamide of molecular weight 5 million or more, and so presumably is not highly anionic (depending on the degree of hydrolysis). Also, it is known to add flocculant powder to a slurry of mine tailings that is being pumped down through a mine to form a backfill. In all these processes, the flocculant powder can be in contact with the slurry for a considerable time before it needs to complete its flocculation effect, and so there is time for the powder to dissolve. Similarly, in JP-A-60282787 a mixture of high molecular weight powdered flocculants is added to an emulsion but again the separation process appears to be sufficiently slow to permit dissolution.
The main difficulty with such methods is that high molecular weight flocculants only dissolve slowly. It has been proposed to treat the polymer particles in various ways, presumably with the intention of accelerating their rate of dissolution. Disclosures of such processes are in, for instance, JP-A-50003974, JP-A-49121309, JP-A-58070807 and JP-A-58089915 and U.S. Pat. No. 4089831. It is also known to dissolve high molecular weight polymeric flocculant from within bags and shaped articles that are immersed for prolonged periods in a flowing suspension, for instance as described in JP-A-53091072 and JP-A-56115605 and EP-A-255283.
All these methods tend to utilise polymer at a rather uncontrollable rate and so may suffer from underdosing or overdosing.
Although it is normally required for the polymer to be in true solution to function effectively there have been some disclosures (e.g., U.S. Pat. Nos. 3,235,490 and 3,021,269) where apparently cross linked and potentially insoluble polymers were homogenised before use as flocculants so as to permit them to form what appears to be a true solution of relatively low molecular weight polymer. Other disclosures of increasing the suitability of polymers by shearing are in U.S. Pat. Nos. 4,705,640 and 4,759,856.
Whereas true solubility is normally required before contact with the aqueous suspension, in U.S. Pat. No. 4,720,346 the polymer is a high molecular weight synthetic polymeric flocculant and performs its flocculating function on the suspended solids while the polymer is still in the form of polymeric particles having a size of below 10 .mu.m. These particles can, if left long enough, be truly soluble in water but preferably the polymer is cross-linked so that they cannot dissolve fully into water.
The very small particle size, of below 10 .mu.m, is essential. Normally it is provided by introducing the polymer particles as a dispersion that has been made by reverse phase polymerisation but in EP-A-326382 it is provided by introducing the polymer in the form of friable aggregates that have been made by bonding the substantially dry polymer particles having a size of below 10 .mu.m with an aqueous liquid and drying the aggregates, whereby the aggregates disintegrate upon addition to water to release the individual polymer particles.
In all coagulation and flocculation processes, there is always the desire to achieve better performance, leading to better clarity or reduced consumption of polymer or both, and/or to achieve simpler techniques of operation.